I am trying to get True Wind from my ST40 Wind instrument.

I am using a Raymarine E85001 SeaTalk/NMEA converter box to take the
NMEA 0183 output from my Garmin GPSMAP175 GPS and send it to the ST40 Wind.

Connecting a PC to the RS232 terminals of the E85001 shows that both
instruments are talking to the E85001, but the ST40 Wind still does not
compute true wind. The GPS is sending NMEA sentences VTG and RMC which
both have boat speed information in them, so I don't understand why the
ST40 won't use it. I've tried choosing different NMEA output settings
on the GPS, and the sentences change slightly, but the ST40 still
ignores the boat speed info.

The ST40 is talking happily to my ST1000+ AutoHelm, and the AutoHelm is
also receiving track data from the GPS (directly, not through the
E85001), so it seems only the ST40 Wind doesn't want to play ball :-(

I've tried contacting Raymarine but it's been nearly two weeks now, and
they still haven't replied to my query, though I live in hope.

Anyone got any ideas what might be going on?

Who.